'Carl Zeiss' and 'Carl Zeiss Jena' are quite different makes. Both are
German but Jena is in the former East Germany. Before the wall came down the
Jena lenses were much cheaper but considered optically inferior. I'm not
sure anyone not using a test bench would notice any difference outside their
bank account.

There is quite a bit of history to this which I'll attempt to summarise.

After the second world war the Carl Zeiss factory in Jena was nationalised
by the Communist East German government. A number of the Carl Zeiss
personnel fled to West Germany to establish a rival company, also called
Carl Zeiss. In the U.S. the West German operation won the right to the Carl
Zeiss name so that all Carl Zeiss Jena lenses exported to America were
labelled "Aus Jena".

Praktica SLRs were manufactured by Pentacon in Dresden. In the 50s and 60s
Pentacon cameras came with a choice of lens options, the basic package was a
body plus a 50/2.8 lens from Meyer Optik called the Domiplan. By paying a
bit more you could upgrade to a superior Carl Zeiss Jena lens like the
50/1.8 Pancolor. At this time most German lens designs from both East and
West were given names by the manufacturers. The Pancolar for instance was
the CZJ version of the Planar, a five element fast lens. Both Zeiss
operations also produced slower pre-war Tessar designs consisting of four
elements in three groups. The Sonnar was another term used by both East and
West German companies to denote a telephoto design also using four elements
in three groups. Flektogon was used by Jena to describe their wide
angle optics, the West German equivalent was Distagon.

With the launch of the B-series cameras in 1979 (of which your BX20 is a
descendent), these old naming conventions were done away with and all of the
East German lenses (and some new Japanese made lenses) for the new bayonet
mount were simply described as Prakticar. For example, what had previously
been the Carl Zeiss Flektogon 35 f2.4 lens in M42 mount became the CZJ
Prakticar 35/2.4 in B mount.

In the late eighties the whole East German camera and optical industry was
consolidated in one company. Carl Zeiss Jena became a brand that was applied
to lenses which had been designed by Pentacon and Meyer, not just those from
the original Zeiss factory. For instance, the Carl Zeiss Jena Prakticar
28/2.8 is just a rebadged Pentacon 28/2.8.

Just when you thought it couldn't get any more confusing, the name was also
licensed to Sigma to produce a the Jenazoom range of zoom lenses available
in most of the manual focus mounts of the late eighties.

After the re-unification of Germany the West German Zeiss operation bought
out the Eastern company and lens production for consumer cameras ceased at
Jena.

Focal lengths and max apertures of the more common "genuine" Carl Zeiss Jena
lenses in B-mount are 20/2.8, 35/2.4, 50/1.4, and 135/3.5. The 28/2.8,
50/1.8 and 135/2.8 "Carl Zeiss Jena P" lenses are rebadged Pentacon lenses.

For more information check out www.praktica-users.com  On this site you can
also see Jon Gibbs's flower close-ups taken with a 135/3.5 Zeiss Jena lens
and bellows which may interest you.

http://www.praktica-users.com/picindex.html